The invention relates to a biological indicator suitable for use in monitoring the efficacy of a sterilization process.
In the sterilization of medical instruments and related hardware it is essential to determine whether a particular batch of articles which has been subjected to a sterilizing environment, such as steam or gas, has in fact been effectively sterilized. A common art recognized technique for carrying out this objective involves subjecting a known number of test microorganisms of resistance to the sterilant employed to the same sterilizing conditions to which the articles in the sterilizer are subjected. At the end of the sterilization cycle, the microorganisms are removed from the sterilizer and exposed to a nutrient culture medium. The microorganisms are incubated for a specified time period and then checked for growth of microorganisms. If no growth of the microorganism occurs, then it is assumed that articles in the sterilizer have been properly sterilized for their intended use. However, if growth has occurred, the articles are determined not to be sterile and should be subjected to a second sterilization cycle.
One commercial embodiment of biological indicators is the spore strip. This device comprises a calibrated spore population inoculated on a paper chromatography grade carrier, and enclosed in a microorganism impermeable glassine envelope. Alternative carrier or envelope materials could be employed. Spore strips require a person to conduct an aseptic transfer of the carrier to presterilized nutrient medium before incubation. Performance standards developed for biological indicators by recognized authorities such as the Association for the Advancement of Medical Instrumentation, are based on glassine spore strip performance characteristics.
A second commercial embodiment of biological indicators is the unitary or self-contained biological indicator. These devices comprise a microorganism impermeable package containing a nutrient medium, and a calibrated spore population inoculated on a suitable carrier material. The aseptic transfer for unitary or self-contained biological indicators is conducted within the microorganism impermeable package as opposed to the case of spore strips where the carrier is removed from the outer packaging materials.
At least two aspects in the construction of biological indicators can influence the kill rate of the calibrated spore population. The first being thermal mass, and the second being the permeation rate of sterilant to the spores. In unitary or self-contained biological indicators, the additional thermal mass provided by the nutrient medium and other packaging materials causes performance characteristics which are different from glassine spore strips. This poses a problem for self-contained biological indicators since the authoritative standards for biological indicator performance are based on glassine spore strips.
One of the first unitary or self-contained biological indicators was described in U.S. Pat. No. 3,346,464. This patent teaches a sterilization indicator in the form of a semi-permeable envelope which is capable of transmitting water and a gaseous sterilizing media without allowing the passage of microorganisms. The envelope contains both test microorganisms and a dehydrated nutrient media. After exposure to a sterilization cycle, the envelope is immersed in warm water. The water permeates the envelope and rehydrates the nutrient media allowing for contact with the test microorganisms, allowing for growth of any remaining viable microorganisms. This structure, however, suffered from a number of design defects and did not achieve wide success. For example, the performance of this device was dependent upon the characteristics of the water in the warm water incubation, which could vary in purity from test to test. In addition, the permeable envelope was vulnerable to entrapping undesirable quantities of sterilizing gas.
More recent improvements to unitary or self-contained biological indicators are illustrated by U.S. Pat. Nos. 3,661,717 and 4,291,122. Both of these patents, however, teach devices which suffer performance anomalies over a range of sterilization temperatures from 270.degree. F.-285.degree. F. (Reich, R. R. and Fitzpatrick, B. G., Journal of Hospital Supply, Processing and Distribution, May-June 1985, vol. 3, No. 3, pp. 60-63). These performance anomalies are due to thermal lag caused by their design and construction (Joslyn L. Sterilization by Heat, in Disinfection, Sterilization and Preservation. Block.S. Eds. Lea & Febiger. Phila., Pa. 1983 pp. 33-34). Common to both the U.S. Pat. Nos. 3,661,717 and 4,291,122 patents is that they both provide for an outer compartment or vial containing a pressure openable inner compartment (i.e., the media ampule) with the microorganisms or spores on their respective carriers located between and adjacent to the two compartments. It is this disposition of the spores between the two compartments and their close juxtaposition with the ampule of media which retards the penetration of the sterilant, augmenting the effect of thermal lag thereby contributing to the performance anomalies of these indicators. Investigations have shown and inferences from the literature (Perkins et al., Applied and Environmental Microbiology Aug., 1981, Vol. 42, No. 2, pp. 383-384) indicate that the sterilization times required at temperatures at or above 270.degree. F. for conventional self-contained biological indicator designs are excessive in comparison to conventional biological indicator designs consisting of spores inoculated onto a paper chromatography grade carrier (or other suitable carrier) and enclosed in a glassine or other suitable material envelope. This can result in "false positive" readings and lead to the reprocessing of an otherwise sterile load and unnecessary equipment downtime. "False negative" readings can also occur at lower sterilization temperatures (240 degrees F. to 245 degrees F.) leading to release as sterile of an underprocessed load whose contents may in fact not be sterile (Reich and Fitzpatrick, ibid).
Specifically, it is an objective of the present invention to provide a self-contained biological indicator whose design circumvents the drawbacks noted above and whose performance characteristics more readily approximate those of a conventional biological indicator than the currently available commercial self-contained biological indicators. This performance over a range of sterilization temperatures may be defined as "The Z-value, measured in degrees of temperature, is the temperature coefficient of microbial destruction and is the time for the D-value or sterilization time of microorganisms to change by a factor of ten." ("Pflung, I. J. and Smith, G. M., The Use of Biological Indicators for Monitoring Wet Heat Sterilization Processes in Sterilization of Medical Products, Eds. E. R. Gaughran and K. Kerluck, Johnson & Johnson 1977).
It is the intent of the device of the present invention to perform with a z-value more closely approximately that of conventional biological indicators and published z-values for B. stearothermophilis. of 18.degree.-20.degree. F. (The Destruction of Bacterial Spores, A. D. Russell, Academic Press, 1982).
Commercially available self-contained biological indicators tested in our laboratories have been found to exhibit Z-values as high as 80 degrees F. as opposed to 18-20 degrees F. for conventional paper strip biological indicators enclosed in glassine envelopes. Proto-types prepared in the manner described by this invention have exhibited Z-values ranging from 20-30 degrees F. and to closely parallel the performance of conventional biological indicators as opposed to the commercially available self-contained biological indicators described above.